The Future of Monolithically Integrated LED Arrays

LEDs are ubiquitous in our electronic lives. They are widely used in notification lighting, flash photography, and light bulbs, to name a few. For displays, LEDs have been commercialized as backlights in televisions and projectors. However, their use in image formation has been limited.

A prototype emissive LED display chip is shown. The chip includes an emissive compass pattern ready to embed into new applications.

A prototype emissive LED display chip is shown. The chip includes an emissive compass pattern ready to embed into new applications.

The developing arena of monolithically integrated LED arrays, which involves fabricating millions of LEDs with corresponding transistors on a single chip, provides many new applications not possible with current technologies, as the LEDs can simultaneously act as the backlight and the image source.

The common method of creating images is to first generate light (using LEDs) and then filter that light using a spatial light modulator. The filter could be an LCD, liquid crystal on silicon (LCoS), or a digital micromirror device (DMD) such as a Digital Light Processing (DLP) projector. The filtering processes cause significant loss of light in these systems, despite the brightness available from LEDs. For example, a typical LCD uses only 1% to 5% of the light generated.

Two pieces are essential to a display: a light source and a light controller. In most display technologies, the light source and light control functionalities are served by two separate components (e.g., an LED backlight and an LCD). However, in emissive displays, both functionalities are combined into a single component, enabling light to be directly controlled without the inherent inefficiencies and losses associated with filtering. Because each light-emitting pixel is individually controlled, light can be generated and emitted exactly where and when needed.

Emissive displays have been developed in all sizes. Very-large-format “Times Square” and stadium displays are powered by large arrays of individual conventional LEDs, while new organic LED (OLED) materials are found in televisions, mobile phones, and other micro-size applications. However, there is still a void. Emissive “Times Square” displays cannot be scaled to small sizes and emissive OLEDs do not have the brightness available for outdoor environments and newer envisioned applications. An emissive display with high brightness but in a micro format is required for applications such as embedded cell phone projectors or displays on see-through glasses.

We know that optimization by the entire LED industry has made LEDs the brightest controllable light source available. We also know that a display requires a light source and a method of controlling the light. So, why not make an array of LEDs and control individual LEDs with a matching array of transistors?

The marrying of LED materials (light source) to transistors (light control) has long been researched. There are three approaches to this problem: fabricate the LEDs and transistors separately, then bond them together; fabricate transistors first, then integrate LEDs on top; and fabricate LEDs first, then integrate transistors on top. The first method is not monolithic. Two fabricated chips are electrically and mechanically bonded, limiting integration density and thus final display resolutions. The second method, starting with transistors and then growing LEDs, offers some advantages in monolithic (single-wafer) processing, but growth of high-quality, high-efficiency LEDs on transistors has proven difficult.

My start-up company, Lumiode (www.lumiode.com), is developing the third method, starting with optimized LEDs and then fabricating silicon transistors on top. This leverages existing LED materials for efficient light output. It also requires careful fabrication of the integrated transistor layer as to not damage the underlying LED structures. The core technology uses a laser method to provide extremely local high temperatures to the silicon while preventing thermal damage to the LED. This overcomes typical process incompatibilities, which have previously held back development of monolithically integrated LED arrays. In the end, there is an array of LEDs (light source) and corresponding transistors to control each individual LED (light control), which can reach the brightness and density requirements of future microdisplays.

Regardless of the specific integration method employed, a monolithically integrated LED and transistor structure creates a new range of applications requiring higher efficiency and brightness. The brightness available from integrated LED arrays can enable projection on truly see-through glass, even in outdoor daylight environments. The efficiency of an emissive display enables extended battery lifetimes and device portability. Perhaps we can soon achieve the types of displays dreamed up in movies.

Multi-Zone Home Audio System

Dave Erickson built his first multi-zone audio system in the early 1990s using C microprocessor code he developed on Freescale MC68HC11 microprocessors. The system has been an important part of his home.

“I used this system for more than 15 years and was satisfied with its ability to send different sounds to the different rooms in my house as well as the basement and the deck,” he says. “But the system needed an upgrade.”

In Circuit Cellar’s January and February issues, Erickson describes how he upgraded the eight-zone system, which uses microprocessor-controlled analog circuitry. In the end, his project not only improved his home audio experience, it also won second place in a 2011 STMicroelectronics design contest.

Several system components needed updating, including the IR remote, graphic LCD, and microprocessor. “IR remotes went obsolete, so the IR codes needed to change,” Erickson says. “The system was 90% hand-wired and pretty messy. The LCD and several other parts became obsolete and the C development tools had expired. Processors had evolved to include flash memory and development tools evolved beyond the old burn-and-pray method.”

“My goal was to build a modern, smaller, cleaner, and more efficient system,” he says. “I decided to upgrade it with a recent processor and LCD and to use real PC boards.”

Photo 1: Clockwise from the upper left, the whole-house system includes the crosspoint board, two quad preamplifiers, two two-zone stereo amplifiers, an AC transformer, power supplies, and the CPU board with the STMicroelectronics STM32VLDISCOVERY board.

Photo 1: Clockwise from the upper left, the whole-house system includes the crosspoint board, two quad preamplifiers, two two-zone stereo amplifiers, an AC transformer, power supplies, and the CPU board with the STMicroelectronics STM32VLDISCOVERY board.

Erickson chose the STMicroelectronics STM32F100 microprocessor and the work incentive of a design contest deadline (see Photo 1).

“STMicroelectronics’s excellent libraries and examples helped me get the complex ARM Cortex-M3 peripherals working quickly,” he says. “Choosing the STM32F100 processor was a bit of overkill, but I hoped to later use it to add future capabilities (e.g., a web page and Ethernet control) and possibly even a simple music server and audio streaming.”

In Part 1 of the series, Erickson explains the design’s audio sections, including the crosspoint board, quad preamplifiers, modular audio amplifiers, and packaging. He also addresses challenges along the way.

Erickson’s Part 1 provides the following overview of the system, including its “analog heart”—the crosspoint board:

Figure 1 shows the system design including the power supplies, front-panel controls, and the audio and CPU boards. The system is modular, so there is flexibility in the front-panel controls and the number of channels and amplifiers. My goal was to fit it all into one 19”, 2U (3.5”) high rack enclosure.

The CPU board is based on a STM32F100 module containing a Cortex-M3-based processor and a USB programming interface. The CPU receives commands from a front-panel keypad, an IR remote control, an encoder knob, RS-232, and external keypads for each zone. It displays its status on a graphic LCD and controls the audio circuitry on the crosspoint and two quad preamplifier boards.

The system block diagram shows the boards, controls, amplifiers, and power supplies.

The system block diagram shows the boards, controls, amplifiers, and power supplies.


Photo 2 shows the crosspoint board, which is the analog heart of the system. It receives line-level audio signals from up to eight stereo sources via RCA jacks and routes audio to the eight preamplifier channels located on two quad preamplifier boards. It also distributes digital control and power to the preamplifiers. The preamplifier boards can either send line-level outputs or drive stereo amplifiers, either internal or external to the system.

My current system uses four line-level outputs to drive PCs or powered speakers in four of the zones. It also contains internal 40-W stereo amplifiers to directly drive speakers in the four other zones. Up to six stereo amplifiers can reside in the enclosure.

Photo 2: The crosspoint board shows the RCA input jacks (top), ribbon cable connections to the quad preamplifiers (right), and control and power cable from the CPU (bottom). Rev0 has a few black wires (lower center).

Photo 2: The crosspoint board shows the RCA input jacks (top), ribbon cable connections to the quad preamplifiers (right), and control and power cable from the CPU (bottom). Rev0 has a few black wires (lower center).

DIYers dealing with signal leakage issues in their projects may learn something from Erickson’s approach to achieving low channel-to-channel crosstalk and no audible digital crosstalk. “The low crosstalk requirement is to prevent loud music in one zone from disturbing quiet passages in another,” he says.

In Part 1, Erickson explains the crosspoint and his “grounding/guarding” approach to transmitting high-quality audio, power, and logic control signals on the same cable:

The crosspoint receives digital control from the CPU board, receives external audio signals, and distributes audio signals to the preamplifier boards and then on to the amplifiers. It was convenient to use this board to distribute the control signals and the power supply voltages to the preamplifier channels. I used 0.1” dual-row ribbon cables to simplify the wiring. These are low-cost and easy to build.

To transmit high-quality audio along with power and logic control signals on the same cable, it is important to use a lot of grounds. Two 34-pin cables each connect to a quad preamplifier board. In each of these cables, four channels of stereo audio are sent with alternating signals and grounds. The alternating grounds act as electric field “guards” to reduce crosstalk. There are just two active logic signals: I2C clock and data. Power supply voltages (±12 and 5 V) are also sent to the preamplifiers with multiple grounds to carry the return currents.

I used a similar grounding/guarding approach throughout the design to minimize crosstalk, both from channel to channel and from digital to analog. On the two-layer boards, I used ground planes on the bottom layer. Grounded guard traces or ground planes are used on the top layer. These measures minimize the capacitance between analog traces and thus minimize crosstalk. The digital and I2C signals are physically separated from analog signals. Where they need to be run nearby, they are separated by ground planes or guard traces.

To find out more about how Erickson upgraded his audio system, download the January issue (now available online) and the upcoming February issue. In Part 2, Erickson focuses on his improved system’s digital CPU, the controls, and future plans.

Real-Time Trailer Monitoring System

Dean Boman, a retired electrical engineer and spacecraft communications systems designer, noticed a problem during vacations towing the family’s RV trailer—tire blowouts.

“In every case, there were very subtle changes in the trailer handling in the minutes prior to the blowouts, but the changes were subtle enough to go unnoticed,” he says in his article appearing in January’s Circuit Cellar magazine.

So Boman, whose retirement hobbies include embedded system design, built the trailer monitoring system (TMS), which monitors the vibration of each trailer tire, displays the

Figure 1—The Trailer Monitoring System consists of the display unit and a remote data unit (RDU) mounted in the trailer. The top bar graph shows the right rear axle vibration level and the lower bar graph is for left rear axle. Numbers on the right are the axle temperatures. The vertical bar to the right of the bar graph is the driver-selected vibration audio alarm threshold. Placing the toggle switch in the other position  displays the front axle data.

Photo 1 —The Trailer Monitoring System consists of the display unit and a remote data unit (RDU) mounted in the trailer. The top bar graph shows the right rear axle vibration level and the lower bar graph is for left rear axle. Numbers on the right are the axle temperatures. The vertical bar to the right of the bar graph is the driver-selected vibration audio alarm threshold. Placing the toggle switch in the other position displays the front axle data.

information to the driver, and sounds an alarm if tire vibration or heat exceeds a certain threshold. The alarm feature gives the driver time to pull over before a dangerous or damaging blowout occurs.

Boman’s article describes the overall layout and operation of his system.

“The TMS consists of accelerometers mounted on each tire’s axles to convert the gravitational (g) level vibration into an analog voltage. Each axle also contains a temperature sensor to measure the axle temperature, which is used to detect bearing or brake problems. Our trailer uses the Dexter Torflex suspension system with four independent axles supporting four tires. Therefore, a total of four accelerometers and four temperature sensors were required.

“Each tire’s vibration and temperature data is processed by a remote data unit (RDU) that is mounted in the trailer. This unit formats the data into RS-232 packets, which it sends to the display unit, which is mounted in the tow vehicle.”

Photo 1 shows the display unit. Figure 1 is the complete system’s block diagram.

Figure 1—This block diagram shows the remote data unit accepting data from the accelerometers and temperature sensors and sending the data to the display unit, which is located in the tow vehicle for the driver display.

Figure 1—This block diagram shows the remote data unit accepting data from the accelerometers and temperature sensors and sending the data to the display unit, which is located in the tow vehicle for the driver display.

The remote data unit’s (RDU’s) hardware design includes a custom PCB with a Microchip Technology PIC18F2620 processor, a power supply, an RS-232 interface, temperature sensor interfaces, and accelerometers. Photo 2 shows the final board assembly. A 78L05 linear regulator implements the power supply, and the RS-232 interface utilizes a Maxim Integrated MAX232. The RDU’s custom software design is written in C with the Microchip MPLAB integrated development environment (IDE).

The remote data unit’s board assembly is shown.

Photo 2—The remote data unit’s board assembly is shown.

The display unit’s hardware includes a Microchip Technology PIC18F2620 processor, a power supply, a user-control interface, an LCD interface, and an RS-232 data interface (see Figure 1). Boman chose a Hantronix HDM16216H-4 16 × 2 LCD, which is inexpensive and offers a simple parallel interface. Photo 3 shows the full assembly.

The display unit’s completed assembly is shown with the enclosure opened. The board on top is the LCD’s rear view. The board on bottom is the display unit board.

Photo 3—The display unit’s completed assembly is shown with the enclosure opened. The board on top is the LCD’s rear view. The board on bottom is the display unit board.

Boman used the Microchip MPLAB IDE to write the display unit’s software in C.

“To generate the display image, the vibration data is first converted into an 11-element bar graph format and the temperature values are converted from Centigrade to Fahrenheit. Based on the toggle switch’s position, either the front or the rear axle data is written to the LCD screen,” Boman says.

“To implement the audio alarm function, the ADC is read to determine the driver-selected alarm level as provided by the potentiometer setting. If the vibration level for any of the four axles exceeds the driver-set level for more than 5 s, the audio alarm is sounded.

“The 5-s requirement prevents the alarm from sounding for bumps in the road, but enables vibration due to tread separation or tire bubbles to sound the alarm. The audio alarm is also sounded if any of the temperature reads exceed 160°F, which could indicate a possible bearing or brake failure.”

The comprehensive monitoring gives Boman peace of mind behind the wheel. “While the TMS cannot prevent tire problems, it does provide advance warning so the driver can take action to prevent serious damage or even an accident,” he says.

For more details about Boman’s project, including RDU and display unit schematics, check out the January issue.

CC278: Serial Displays Save Resources (BMP Files)

In Circuit Cellar’s September issue, columnist Jeff Bachiochi provides his final installment in a three-part series titled “Serial Displays Save Resources.” The third article focuses on bitmap (BMP) files, which store images.

Photo1

A BMP file has image data storage beginning with the image’s last row. a—Displaying this data as stored will result in an upside-down image. b—Using the upsidedown=1 command will rotate the display 180°. c—The mirror=1 command flips the image horizontally. d—Finally, an origin change is necessary to shift the image to the desired location. These commands are all issued prior to transferring the pixels, to correct for the way the image data is stored.

LCDs are inexpensive and simple to use, so they are essential to many interesting projects, Jeff says. The handheld video game industry helped popularize the use of LCDs among DIYers.

Huge production runs in the industry “made graphic displays commonplace, helping to quickly reduce their costs,” Jeff says. “We can finally take advantage of lower-cost graphic displays, with one caveat: While built-in hardware controllers and drivers take charge of the pixels, you are now responsible for more than just sending a character to be printed to the screen. This makes the controllers and drivers not work well with the microcontroller project. That brings us to impetus for this article series.

“In Part 1 (‘Routines, Registers and Commands,’ Circuit Cellar 276, 2013), I began by discussing how to use a graphic display to print text, which, of course, includes character generation. In essence, I showed how to insert some intelligence between a project and the display. This intermediary would interpret some simple commands that enable you to easily make use of the display’s flexibility by altering position, screen orientation, color, magnification, and so forth.

“Part 2 (‘Button Commands,’ Circuit Cellar 277) revealed how touch-sensitive overlays are constructed and used to provide user input. The graphic display/touch overlay combination is a powerful combination that integrates I/O into a single module. Adding more commands to the interface makes it easier to create dynamic buttons on the graphic screen and reports back whenever a button is touched.

The prototype PCB I used for this project mounts to the reverse side of the thin-film transistor (TFT) LCD. The black connector holds the serial and power connections to your project. The populated header is for the Microchip Technology MPLAB ICD 3 debugger/programmer.

“Since I am using a graphic screen, it makes sense to investigate graphic files. This article (Part 3, ‘BMP Files,’ Circuit Cellar 277) examines the BMP file makeup and how this relates to the graphic screen.”

To learn more about the BMP graphical file format and Jeff’s approach to working with a graphic icon’s data, check out the September issue.

 

DIY 10.1˝ Touchscreen Home Control System

Domotics (home automation) control systems are among the most innovative and rewarding design projects creative electrical engineers can undertake. Let’s take a look at an innovative Beagle Board-based control system that enables a user to control lights with a 10.1˝ capacitive touchscreen.

Domotics control system

The design features the following modules:

• An I/O board for testing purposes
• An LED strip board for controlling an RGB LED strip
• A relay board for switching 230-VAC devices
• An energy meter for measuring on/off (and also for logging)

ELektor editor and engineer Clemens Valens recently interviewed Koen van Dongen about the design. Van Dongen describes the system’s electronics and then demonstrates how to use the touchscreen to control a light and LED strip.

As Valens explains suggests, it would be a worthwhile endeavor to incorporate a Wi-Fi connection to enable cellphone and tablet control. If you build such system, be sure to share it with our staff. Good luck!

CircuitCellar.com is an Elektor International Media website.